In an article for The Wall Street Journal on Merce Cunningham's decision to leave his works to a trust and disband his company after a farewell tour after his death, Terry Teachout gives a clear analogy about how choreography is transmitted and why it changes over time, when there's no deliberate attempt to do so:
You can write down a piano concerto, but you can’t write down a ballet. Dance notation is so complex and inexact that no choreographer has ever used it to create a new piece from scratch. In fact, most choreographers and dancers don’t even know how to read dance notation, much less write it. Instead of sitting at a desk and writing down the steps of a new dance, a choreographer makes them up on the spot in a studio and personally teaches them to his dancers, who then perform them from memory on stage.
No other art form works this way. Imagine that instead of writing down his Fifth Symphony, Beethoven had taught it to the members of the Vienna Philharmonic by playing it on the piano over and over again until each musician knew his own part by heart. Now suppose that the Philharmonic liked the Fifth Symphony so much that it continued to perform the piece for the next two centuries, with each succeeding generation of players learning the score by rote from its predecessors. Ask yourself this: What would Beethoven’s Fifth sound like today? Would it still sound the same way it did in 1808, or would it have undergone dramatic changes in the process of being transmitted by ear from musician to musician? Or might it have been forgotten altogether?